1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit for use in a high frequency circuit of a receiver or the like.
2. Description of the Related Art
In recent years, digitalized mobile telephone systems have been developed because of the realization of their ability for accurate communication. The mobile telephone system is based on a multi-channel access (MCA) system in which one of the communication channels corresponding to a suitable frequency is selected according to several conditions.
In a conventional analog mobile telephone system, a channel change-over during a call inevitably involves the instantaneous break of the voice signal. However, when the quality of the speech becomes degraded, an operator often switches the current channel to another suitable communication channel to avoid this degradation. With a digitalized mobile telephone system, since an effect of an instantaneous break is remarkably reduced, the channel change-over can be conducted rather successfully for the purpose of enhancing frequency utilization efficiency. The channel change-over during the call is carried out both in the case where communication channel change-over is desired in an identical area and in the case where change-over to a base station called "hand-off" is desired.
In more detail, when a mobile station terminal moves during a call, the communication of the terminal with an initially connected base station becomes difficult. The operation to search for a new base station closest thereto and the switching connection thereto is termed "hand-off". There has recently been proposed a hand-off system of a mobile-station-initiative type in which, during a call, a mobile station measures the level of a signal received from a nearby base station and reports the measured level to the base station, while the base station determines a base station to be switched on the basis of the received report. In the hand-off system, a mobile station can switch to a different carrier frequency during an idle time other than the communication time, monitor the level of electromagnetic waves received from another base station, and select an optimum one of the base stations during the channel change-over. This feature one of the major advantages of a mobile telephone system of a time division multiple access (TDMA) type. This hand-off system is highly effective in realizing efficient and reliable control for small sized zones.
FIG. 9 is a diagram for explaining the hand-off operation of the mobile telephone system of the TDMA communication type from a base station A to a base station B. In the drawing, a control station 91 is connected to the base stations A and B which can be connected to a mobile station 90. The mobile station 90 measures the electric field strength of a received signal strength indicator (RSSI) or a bit error rate (BER) of a signal received on another communication channel from another base station during the idle communication time, and reports the measured result to the base station A being initially connected to the mobile station. When the RSSI is larger than the level of the signal received on its own communication channel or when the bit error rate is small, the base station A transmits a hand-off signal to the mobile station 90. The mobile station 90, when receiving the hand-off signal, switches the current channel to the communication channel of the base station B to maintain subsequent good communications.
For the purpose of keeping constant the level of the received signal, the mobile station contains an automatic gain control (AGC) circuit provided in a high frequency circuit or the like of the receiver side. The AGC circuit functions to keep a constant gain by feeding the output of the high frequency amplification circuit back thereto.
Referring to FIG. 10, there is shown a block diagram of an arrangement of a prior art AGC circuit, in which an intermediate frequency received from an input terminal amplified at an intermediate frequency amplifier 106, subjected at a band pass filter 107 to a removal of signal components in a predetermined frequency band, and then output from an output terminal 108. In order to keep constant the output of the filter 107, the intermediate frequency output of the band pass filter 107 is applied to an AGC circuit 100 which in turn controls the gain of the intermediate frequency amplifier 106.
More specifically, in the AGC circuit 100, the intermediate frequency signal received from the band pass filter 107 is detected at a level detector 101 and then supplied to an error amplifier 104 through a time constant circuit 102 which provides a level signal of the smoothed intermediate frequency signal after a predetermined time lag determined by the time constant circuit 102. The error amplifier 104 finds a difference between the received level signal of the input intermediate frequency signal, i.e., a signal having a voltage based on electric charges accumulated in a capacitor 102-3 and a reference voltage level supplied from a reference voltage source 103 and sends a signal corresponding to the difference to the intermediate frequency amplifier 106 to control the gain of the intermediate frequency amplifier 106. As a result, the gain of the intermediate frequency signal can be controlled to be constant.
Meanwhile, the mobile station is monitoring for the aforementioned hand-off operation, that is, receiving a signal from another base station not communication-connected with the mobile station during the idle time for switching to the communication channel of the other base station. During this monitoring operation, the mobile station measures the RSSI or BER of a signal received from another base station during the idle time other than its substantial communication time.
After monitoring in the idle mode, when it is desired for the mobile station to switch to a receive-signal carrier frequency to be returned to its own communication operation, since the AGC circuit is set in such a monitor mode that the RSSI is measured in the idle condition, the mobile station cannot be returned immediately to its previous communication state. In other words, the output of the AGC circuit transient.
Therefore, the prior art AGC circuit has had such a problem that, even after the mobile station switches own communication channel, it cannot be put immediately in its previous stationary communication state but it requires a certain time to return to the stationary communication state.